Rechargeable secondary batteries have been actively studied for the development of digital cameras, mobile phones, lap-top PCs, power tools, electric bicycles, electric vehicles, hybrid vehicles, and large-capacity power storage devices.
Particularly, lithium secondary batteries have a relatively high energy density per unit weight and are rapidly chargeable when compared with other secondary batteries such as lead storage batteries, nickel-cadmium batteries, nickel-hydrogen batteries and nickel-zinc batteries, and thus, their use is rapidly increasing.
Such a lithium secondary battery has a working voltage of 3.6 V or more and may be used as power sources for portable electronic devices or used for electric vehicles, hybrid vehicles, power tools, electric bicycles, power storage devices, uninterruptible power supplies (UPSs), and the like, in which a plurality of batteries are connected in series or in parallel to each other, and an high output is required.
Since the lithium secondary battery has a working voltage greater three times than that of a nickel-cadmium battery or a nickel metal hydride battery and has an excellent energy density per unit weight, its use is rapidly increasing.
The lithium secondary battery may be classified into a lithium ion battery using a liquid electrolyte and a lithium ion polymer battery using a polymer solid electrolyte according to types of electrolyte.
Also, the lithium ion polymer battery may be classified into a fully solid lithium ion polymer battery which does not contain any electrolytic solution at all and a lithium ion polymer battery which uses a gel-type polymer electrolyte containing an electrolytic solution according to kinds of polymer solid electrolyte.
In case of the lithium ion battery using the liquid electrolyte, most of the batteries are used in a sealed form by using a cylindrical or square metal can as a container.
As described above, since the secondary battery using the metal can as the container has a fixed shape, there is a disadvantage in that the design of the electrical product using the metal can as a power source is limited, and it is difficult to reduce its volume.
Thus, a pouch type secondary battery in which an electrode assembly and an electrolyte are put into a pouch packing material made of a film and then sealed has been developed and is being used.
However, if the lithium secondary battery is overheated, since there is a risk of explosion, securing safety is one of important tasks.
The overheating of the lithium secondary battery is caused by various causes, and on of them is a flow of overcurrent beyond the limit through the lithium secondary battery.
When the overcurrent flows, the lithium secondary battery generates heat by Joule's heat, and thus, an internal temperature of the battery rapidly increases.
Also, the rapid temperature increase brings about a decomposition reaction of an electrolyte solution to cause a thermal running phenomenon, resulting in explosion of the battery.
The overcurrent occurs when rush current is applied to the battery due to insulation breakdown between a positive electrode and a negative electrode, which is caused by penetration of a pointed metal object through the lithium secondary battery or shrinkage of a separator interposed between the positive electrode and the negative electrode, or due to an abnormal condition of an external charging circuit or load being connected to the outside.
Thus, to protect the lithium secondary battery from an abnormal situation such as an occurrence of the overcurrent, the battery is used in combination with a protection circuit.
In general, the protection circuit includes a fuse device that irreversibly disconnects a line through which charging or discharging current flows when the overcurrent occurs.
A conventional battery pack and a connecting bar applied thereto are disclosed in Korean Patent Publication No. 10-2013-0080023.
However, the conventional connecting bar has a limitation in safety in that the battery is damaged by spark generated when an external short-circuit of the battery occurs, and the battery is ignited.